1. Field of the Invention
The present invention relates to an ink-jet recorder which carries out a recording operation by squirting an ink droplet from a nozzle by means of the pressure of a bubble resulting from generation of heat by a heating element. Particularly, the present invention relates to an ink-jet recorder having nozzles for squirting ink having different colors or concentrations.
2. Description of the Related Art
Commercialization of an ink-jet recording system is proceeding for reasons that the ink-jet recording system can perform high-speed recording without substantial noise, can produce print on ordinary paper directly, and can be reduced in size because it does not require fixing processing.
The ink-jet recording system comprises a system which uses an electromechanical transducer as means for squirting an ink droplet from a nozzle, and which squirts an ink droplet by means of movement resulting from the mechanical deformation of the electromechanical transducer corresponding to an input signal. The ink-jet recording system further comprises a so-called thermal ink-jet system which uses an electro-thermal transducer (a heating resistor), and which squirts an ink droplet by means of the pressure of a bubble developed on the heating element when the heating element produces heat as a result of receipt of an applied voltage pulse.
FIGS. 14A to 14C show one example of a conventional thermal ink-jet head. FIG. 14A is a cross-sectional view of the thermal ink-jet head which is taken in the axial direction of a channel; FIG. 14B is a plan view of the thermal ink-jet head taken along line B--B shown in FIG. 14A; and FIG. 14C is a front view of the thermal ink-jet head as viewed from a nozzle. In the drawings, reference numeral 1 designates a channel substrate, 2 designates a heating resistor substrate, 3 is a channel, 4 is a common liquid chamber, 5 is a nozzle, 6 is an unetched area, 7 is a heating resistor, 8 is an insulating layer, 9 is a thick-film insulating layer, 10 is a first indentation, 11 is a second indentation, 12 is a partition, 13 is an ink droplet, and 14 is an ink feed port. FIGS. 14A to 14C show a thermal ink-jet head disclosed in Unexamined Japanese Patent Application No. Hei-5(1993)-155020 as one example.
The channel 3 and the common liquid chamber 4 are formed in the channel substrate 1 by anisotropic etching, and an opening of the channel 3 acts as the nozzle 5. The channels 3 are formed with pitches Pn, and they are separated from each other by the partitions 12. The unetched area 6 is present between the channel 3 and the common liquid chamber 4. The common liquid chamber 4 is formed so as to pass through the channel substrate 1, and a through-hole of the common liquid chamber serves as the ink feed port 14.
The heating resistors 7 are formed in the heating resistor substrate 2, and electrodes (not shown) and protective films (not shown) for feeding a drive signal to the heating resistors 7 are formed on the heating resistor substrate 2. The insulating layer 8 and the thick-film insulating layer 9 are further formed on the heating resistor substrate 2. The insulating layer 8 and the thick-film insulating layer 9 are removed from the top of the heating resistor 7, so that the first indentation 10 is formed. The second indentation 11 is formed in the thick-film insulating layer 9 in order to connect the channel 3 to the common liquid chamber 4. These two substrates, i.e., the channel substrate 1 and the heating resistor substrate 2 are cemented together, and they are cut into individual head chips, whereby a thermal ink-jet head is manufactured.
The ink fed from the ink feed port 14 to the common liquid chamber 4 is introduced into the channel 3 which is an ink flow path via the second indentation 11 formed in the thick-film insulating layer 9. The ink is squirted from the nozzle 5 to a recording medium in the form of the ink droplet 13 by means of the pressure of a bubble which is formed in the first indentation 10 as a result of generation of heat by the heating resistor 7.
FIG. 15 is a detailed cross-sectional view of a surrounding area of a heating resistor of one example of a conventional ink-jet recording head, and FIG. 16 is a top view of the surrounding area of the heating resistor shown in FIG. 15. In the drawings, the same elements as those shown in FIG. 14 are assigned the same reference numerals, and their explanations will be omitted here. Reference numeral 21 designates a common electrode, 22 designates an individual electrode, 23 designates a Ta layer, 24 designates a Si.sub.3 N.sub.4 layer, 25 designates a heating region, 26 designates a low-resistance area, 27 designates a first glass layer, 28 designates a second glass layer, 29 designates a SiO.sub.2 layer, 30 designates a Si substrate, and 31 and 32 designate through-holes.
After the SiO.sub.2 layer 29, which acts as a thermal storage layer, has been formed on the Si substrate 30, a polycrystalline silicon layer which acts as a heating resistor is formed on the SiO.sub.2 layer 29. The polycrystalline silicon layer is of high resistance, and hence it is necessary to reduce the resistance to a value suitable for the heating resistor. In order to cause only a predetermined area which produces a bubble to generate heat, it is necessary to form the low-resistance area 26 by reducing the resistance of an area other than the area of the polycrystalline layer in which the heating area 25 is to be formed, that is, the resistance of an area of the polycrystalline silicon layer which becomes electrodes extending from the heating electrode 25 to the common electrode 21 and the individual electrode 22. Impurity ions (P or As ions) are implanted into that area (i.e., ion implantation) in order to reduce the resistance of the area.
In FIG. 16, after having been laid, the polycrystalline silicon is patterned, so that a polycrystalline silicon layer is formed. The resistance of the polycrystalline silicon layer is reduced to a suitable value by ion implantation, whereby the heating area 25 is formed. In order to connect the heating area 25 to the common electrode 21 and the individual electrode 22, the resistance of the polycrystalline silicon layer is further reduced by carrying out the ion implantation again. As a result, the low-resistance area 26 is formed.
The first glass layer 27 which serves as an interlayer insulating film is formed over the low-resistance area 26. The through-holes 31, 32 are formed in this first glass layer 27 so as to electrically connect the low-resistance area 26 to the common electrode 21 and the individual electrode 22. Subsequently, the Si.sub.3 N.sub.4 layer 24 which serves as an insulating layer and the Ta layer 23 which serves as a protective metal layer are formed, in that order, on the heating area 25. To provide electrical energy to the heating area 25, the common electrode 21 and the individual electrode 22 which are formed from aluminum are patterned on the first glass layer. At this time, the common electrode 21 and the individual electrode 22 are connected to the polycrystalline silicon layer 26 via the through-holes 31 and 32 formed in the first glass layer 27. Then, the second glass layer 28, the insulating layer 8, and the thick-film insulating layer 9 are formed on the substrate in that order.
In the thermal ink-jet head manufactured in the manner as previously described, the size of the ink droplet squirted from the nozzle depends on so-called flow path parameters, that is, the size and location of the heating resistor and the length and width of the flow path, as well as the physical properties of the ink. Therefore, in the case of an ink-jet recorder which records a color image using a plurality of ink-jet heads for squirting ink having different colors, or in the case of an ink-jet recording system which reproduces gradations using a plurality of ink-jet recorders for squirting ink having different concentrations, it is necessary to change the flow path parameters of the thermal ink-jet head in order to cause an appropriate quantity of ink droplet to be squirted.
Unexamined Japanese Patent Application No. Sho-57(1982)-87960 discloses a longitudinally mounted ink-jet head, wherein the size of a heating element is changed in order to correct the difference between the amounts of ink droplet to be squirted due to the difference in ink pressures which nozzles receive. As disclosed in that application, it is most effective to change the size of the heating resistor of the flow path parameters in order to change the size of the ink droplet.
However, if the heating resistors are different from each other in size, the energy required to produce a bubble on the heating resistor and the resistance value of the heating resistor become different. As a result, it is necessary to change the voltage value and the pulse width of the voltage pulse used to actuate the ink-jet head. Accordingly, it is necessary to provide the ink-jet recorder with a high-speed control circuit which has a voltage corresponding to each of the nozzles (the heating resistor) and ink-jet heads, or a plurality of power supplies for feeding a plurality of voltages, which adds to the cost of the ink-jet recorder.
It may be conceived that the pulse width of the voltage pulse is changed while the voltage is maintained. FIG. 5 shows the relationship between the voltage applied to the heating resistor and the amount of ink droplet when the width of the applied voltage pulse is changed. In the drawing, reference numeral 41 designates a flat region. Curves "a" to "d" designate pulse widths of 2.5 .mu.s, 3 .mu.s, 3.5 V .mu.s, and 4 .mu.s, respectively.
As designated by the curves "a" and "b", if the pulse having a relatively narrower pulse width is applied to the heating resistor, the curves have the flat region 41 in which the amount of ink droplet to be squirted does not depend on the applied voltage. To make the amount of ink droplet to be squirted less susceptible to the influence of variations in the supply voltage, it is desirable to set the drive voltage of the ink-jet recorder in that flat region 41. If the pulse width becomes wider, the curves do not have the flat region 41 in which the size of an ink droplet to be squirted is not affected by voltage variations, as designated by the curves "c" and "d". If the supply voltage changes due to environmental conditions, or the like, the size of ink-droplet to be squirted may greatly change.
Conversely, if the pulse width becomes excessively narrow, the bubble will not arise. In short, the layer formed on the heating resistor is made up of a plurality of layers in order to protect the heating resistor from cavitation damage resulting from the disappearance of the bubbler and hence that layer has a certain degree of thickness. Because of the heat capacity of that layer, temperature variations in the surface which is in contact with the ink cannot respond to the voltage pulse. As a result, the temperature of that surface fails to reach the temperature necessary to produce a bubble, and hence the bubble does not arise, which in turn prevents the squirt of the ink droplet. As described above, it is desirable to actuate the ink-jet nozzle head using the optimum pulse width in order to stably squirt the ink droplet. Hence, it is not desirable to change the pulse width for each ink-jet head. The change of the pulse width renders the drive circuit complicated.
Further, it is conceived that one ink-jet head is divided in such a way as to be assigned to ink of a plurality of colors in order to reduce the number of ink-jet heads in the ink-jet recorder. In this case, it is still further undesirable to feed voltage pulses having different amplitudes and widths to heating resistors for the ink of different colors in one head in view of an increase in the number of wires and the configuration of the drive circuit.
If the heating resistors which require different energies are actuated by the same voltage pulse (the same voltage and the same pulse width), the following problems will arise. If the pulse conditions are set so as to provide sufficient energy to the heating resistors which require high energy, the energy conditions become much excessive for the heating resistors which require low energy. As a result, the life of the heating resistors becomes shortened, or changes in the characteristics of the heating resistors due to the scorch of the ink caused when the ink is squirted are accelerated. On the other hand, if the pulse conditions are set so as to be suitable for the heating resistors that require low energy, the amount of ink droplet squirted by the heating resistors that require high energy may become apt to be affected by voltage variations, or the heating resistors may not squirt ink because of burns of the ink adhered to the heating resistors as a result of the squirting of the ink for a long period of time.
Unexamined Japanese Patent Application No. Hei-3(1991)-224743 discloses a bubble generating area which is smaller than a heating area. As will be described later, it becomes possible to reduce the amount of ink droplet as a result of reductions in the size of the bubble generating area. However, the use of such a configuration in the above described patent application is only intended to reduce cavitation damage. The above described patent application fails to describe or teach the acquisition of a suitable amount of ink droplet with regard to the ink of a plurality of colors or concentrations as previously described.